The detection of neutrons in the presence of a large gamma photon flow has always been difficult. One of the processes used in the prior art consists of using detectors such as .sup.10 BF, .sup.3 He proportional counters, intrinsically disturbed by high X or gamma dose rates and surrounding them with a shield which is effective with respect to said radiation.
Another process consists of using fission chambers insensitive to said radiation, but which have a limited efficiency for neutrons.
It is often necessary to simultaneously detect emissions of neutrons and X or gamma photons from a radiation source. This is generally carried out in the prior art by using two detectors, one for the neutrons and the other for the X or gamma photons.
The prior art in this field is e.g. illustrated by FR-A-2 317 668 and DE-A-1 564 271. These two documents relate to an apparatus effectively using two detectors, one for the neutrons and the other for the photons, both producing visible photons by scintillation, said two detectors being associated with the same photomultiplier.
The problem of this selective detection consequently consists of using means making it possible, from among the scintillations taken into account by the photomultiplier, to obtain a selection of those due to the neutrons and of those due to the X or gamma photons.
In the prior art, the selection of the scintillations respectively relative to said two radiation categories takes place at the scintillator outlet by a processing of the signal using the special, distinctive characteristics of said same scintillations as a function of whether they are due to neutrons or photons.
Essentially this discrimination takes place either on the rise time of the pulses (in FR-A-b 2 317 668), or on the decay time differences of the pulses emitted by the two scintillators (DE-A-1 564 271). In both cases, this signal processing phase leads to complex spectrometric electronics and to expensive equipment.
Another document, "United States Statutory Invention Registration", No. H590, described a scintillation detector sensitive both to neutrons and to gamma rays as a result of the presence of two scintillators, namely an external scintillator for the thermal neutrons and an internal scintillator for the fast neutrons and gamma photons.
However, in the aforementioned prior art, discrimination takes place in the output electronic system by pulse shape measurements and the counting of the pulses one by one following selection, i.e. using complex and expensive equipment.
It is for this reason that in said detector, the scintillator sensitive to the gamma photons is a plastic scintillator which, by its very nature, is unable to supply photoelectric peaks usable in amplitude spectrometry.